There is a need to detect traces of substances of interest such as explosives, narcotics and chemical warfare agents. Reliable and accurate identification is critical. Analysis may be performed using spectrometers, such as ion mobility spectrometers and/or mass spectrometers.
Mass spectrometry works by ionizing chemical compounds to generate charged molecules or molecule fragments and measuring their mass-to-charge ratios. In a typical mass spectrometry procedure ions are separated according to their mass-to-charge ratio, typically by accelerating them and measuring the degree to which they are deflected by an applied electric or magnetic field. Some mass spectrometers operate using ion traps. Mass spectra reflect the relative abundance of detected ions as a function of their mass-to-charge ratio. The ions can be identified by comparing known masses to the identified masses or by comparing obtained spectra to known spectra. Ions of the same mass-to-charge ratio will undergo the same amount of deflection, but a single mass-charge ratio may be associated with a number of different species of ions.
Ion mobility spectrometers (IMS) can identify material from a sample of interest by ionizing the material (e.g., molecules, atoms) and measuring the time it takes the resulting ions to travel a known distance under a known electric field. This is known as time of flight ion mobility spectrometry—TOFIMS. Each ion's time of flight can be measured by a detector, and the time of flight is associated with the ion's mobility. An ion's mobility relates to its mass and geometry. Therefore, by measuring the time of flight of an ion in the detector it is possible to infer an identity for the ion. These times of flight may be displayed graphically or numerically as a plasmagram. Other kinds of ion mobility spectrometry also exist. For example, in differential ion mobility spectrometry ions are selected based on the dependency of the ion mobility on electric field strength. To do this, ions may be subjected to varying electric field strengths selected so that only ions having a selected differential mobility are able to pass though the spectrometer. For example, in field asymmetric ion mobility spectrometry ions are separated by the application of a high-voltage asymmetric waveform at radio frequency (RF) combined with a DC voltage. As the electric field varies, depending on the ratio of the high-field and low-field mobility of an ion, it will migrate toward one or the other electrode. Only ions with specific differential mobility will pass through the device.
Other ion mobility techniques use a flow of carrier gas which carry ions between electrodes. Ions can be deflected from their path in the flow of gas by an electric field applied between the electrodes across the direction of (transverse to) the flow of carrier gas. By scanning the strength of the applied field ions having different mobility can be selected.